parents in this situation are the worst people to talk to about it. Likely they are part of the dynamic that helped to encourage his behavior so that's why he is the way he is now in adulthood. whether they encouraged it actively or they did it by inaction - that is, not discouraging the bullying, either way his reward is that he was able to have free reign to bully others and knew his parents would not stop it.

They never saw it as a problem leading up to this point, why would you think they have any kind of wise insight as to what to do about it? They literally are ignorant to the fact that his behavior is bullying and abusive.